The T/t complex in the mouse contains a number of mutations that are lethal during embryonic development, and that also alter sperm function and morphology. Serologically definable abnormal cell surface antigens are specified by these mutations. We are attempting to characterize these complex antigens as to their individual serological specificities, and their biochemical structure. We are also characterizing the representation of these abnormal antigens and their normal counterparts on developing embryos, and attempting to assay their functional role.